The invention relates to electro-optical systems used to perform extremely accurate measurement of changes in either length or optical path length, e.g., interferometry systems. More particularly, the invention relates to an apparatus for use with an interferometry system in which the apparatus transforms a single frequency, linearly polarized Laser beam into a beam with two frequency components that are orthogonally polarized.
The use of optical interferometry to measure changes in either length, distance, or optical path length has grown significantly due not only to technological advances in lasers, photosensors, and microelectronics but also to an ever increasing demand for high precision, high accuracy measurements [cf. N. Bobroff, xe2x80x9cRecent advances in displacement measuring interferometry,xe2x80x9d Meas. Sci. Technol., 4(9), 907-926 (1993)]. The prior art interferometers can be generally categorized into two types based on the signal processing technique used, i.e., either homodyne or heterodyne. The interferometers based on the heterodyne technique are generally preferred because (1) they are insensitive to low frequency drift and noise and (2) they can more readily have their resolution extended. Within the heterodyne type of interferometers, of particular interest are the ones based on the use of two optical frequencies.
In the prior art two-optical frequency heterodyne interferometers, the two optical frequencies are produced by one of the following techniques: (1) use of a Zeeman split laser, see for example, Bagley et al., U.S. Pat. No. 3,458,259, issued Jul. 29, 1969; G. Bouwhuis, xe2x80x9cInterferometrie Mit Gaslasers,xe2x80x9d Ned. T. Natuurk, 34, 225-232 (August 1968); Bagley et al., U.S. Pat. No. 3,656,853, issued Apr. 18, 1972; and H. Matsumoto, xe2x80x9cRecent interferometric measurements using stabilized lasers,xe2x80x9d Precision Engineering, 6(2), 87-94 (1984); (2) use of a pair of acousto-optical Bragg cells, see for example, Y. Ohtsuka and K. Itoh, xe2x80x9cTwo-frequency Laser Interferometer for Small Displacement Measurements in a Low Frequency Range,xe2x80x9d Applied Optics, 18(2), 219-224 (1979); N. Massie et al., xe2x80x9cMeasuring Laser Flow Fields With a 64-Channel Heterodyne Interferometer,xe2x80x9d Applied Optics, 22(14), 2141-2151 (1983); Y. Ohtsuka and M. Tsubokawa, xe2x80x9cDynamic Two-frequency Interferometry for Small Displacement Measurements,xe2x80x9d Optics and Laser Technology, 16, 25-29 (1984); H. Matsumoto, ibid.; P. Dirksen, et al., U.S. Pat. No. 5,485,272, issued Jan. 16, 1996; N. A. Riza and M. M. K. Howlader, xe2x80x9cAcousto-optic system for the generation and control of tunable low-frequency signals,xe2x80x9d Opt. Eng., 35 (4), 920-925 (1996); (3) use of a single acousto-optic Bragg cell, see for example, G. E. Sommargren, commonly owned U.S. Pat. No. 4,684,828, issued Aug. 4, 1987; G. E. Sommargren, commonly owned U.S. Pat. No. 4,687,958, issued Aug. 18, 1987; P. Dirksen, et al., ibid.; or (4) use of two longitudinal modes of a randomly polarized HeNe laser, see for example, J. B. Ferguson and R. H. Morris, xe2x80x9cSingle Mode Collapse in 6328 xc3x85 HeNe Lasers,xe2x80x9d Applied Optics, 17(18), 2924-2929 (1978).
As for the prior art use of a Zeeman split laser to produce the two optical frequencies, this approach is only applicable to certain lasers (e.g., HeNe) and limits the frequency difference between the two optical frequencies to about 2 MHz. This imposes a limit on the maximum rate of change of the length or optical length being measured. In addition, the available power from a Zeeman split laser is less than 500 microwatts, which can be a serious limitation when one laser source must be used for the measurement of multiple axes, such as three to six axes.
The acousto-optical modulator with a single acousto-optical Bragg cell of Sommargren, commonly owned U.S. Pat. No. 4,684,828 and of Dirksen, et al., ibid., and the acousto-optical modulator with two acousto-optical Bragg cells of Dirksen, et al., ibid., are based on normal Bragg diffraction in both non birefringent and birefringent Bragg cells. The normal Bragg diffraction generates a diffracted beam wherein the state of linear polarization of the diffracted beam is the same state of linear polarization as the incident, undiffracted beam. However, the objectives of the heterodyne interferometry are usually best served when the two optical beam components from an acousto-optical modulator are frequency shifted one with respect to the other, orthogonally polarized, and collinear. The process of converting the output beam components generated by a normal Bragg diffraction acousto-optical modulator, i.e., two non collinear beams in the same linear polarization state into two collinear beams in orthogonally polarized beams, has had an efficiency significantly less than 100%.
Accompanying the increasing demand for improved hiqh precision, high accuracy distance measurements is a demand to increase the number of axes being measured with distance measuring interferometry. The demand to increase the number of axes being measured translates to either increasing the number of laser source-acousto-optical modulator units, increasing the power of the laser source, and/or increasing the conversion efficiency with respect to power of the two frequency heterodyne source. An increase in the conversion efficiency is clearly an attractive option from a commercial point of view.
The present invention relates to an apparatus for providing light beams of orthogonal states of polarization and of different frequency for use in precision metrology applications such as in the measurement of length or length changes using interferometric techniques. The light beams of orthogonal states of polarization are typically parallel but may beneficially have a predetermined angle of divergence or convergence between them. Different embodiments of the invention are disclosed in the form of optical devices for efficiently converting an input optical beam comprising two components having differing frequency profiles, the same states of linear polarization, and directions of propagation differing by a small predetermined angle from a light source, typically comprising a single frequency laser and acousto-optical modulator, to an output beam having two principal, typically parallel, output beams of differing orthogonal states of polarization, one principal output beam comprising substantially the same frequency components as one of the input beam components and another principal output beam comprising substantially the same frequency components as another of the input beam components. The frequency profiles of the input beam components are typically different but may beneficially have the same frequency profiles for some applications. The energy flux profiles of the principal output beams may be spatially separated, partially coextensive, or substantially coextensive in accordance with the details of particular device embodiments. The input beam is introduced to a series of at least one phase retardation plate where it experiences phase retardations via optical birefringence of the at least one phase retardation plate to form two sets of orthogonally polarized internal beam components diverging by a small angle. The two sets of orthogonally polarized internal beam components subsequently become four external beams two of which, the principal ones, are available outside of the at least one phase retardation plate for use in anticipated downstream applications. The remaining two of the four output beams are typically reduced to nominally zero intensities compared to the intensity of the input beam so as to achieve a high efficiency conversion of the input beam into the principal output beams, thus rendering the two output beams with reduced intensities spurious. Spatial filtering may be used to further control any negative impact of the spurious beams.
Depending on the specific embodiment, progenitor beam components of selected ones of the external beams are either intercepted within or outside the series of at least one phase retardation plate so that the selected ones of the external beams are rendered typically parallel by a collimating means. The collimating means can be in the form of internal reflecting and/or integral refracting surfaces and/or external elements. However, if desired, the selected ones of the external beams can be non-parallel such that they have a predetermined angle of divergence or convergence between them.
The degree of overlap or spatial separation between the energy flux profiles of the principal, linearly-orthogonally polarized, external beams is controlled by various internal reflecting and refracting properties of the series of at least one phase retardation plate including the birefringence and optical properties of the material of the series of at least one phase retardation plate, the length of the physical path of travel experienced by the internal beam components, and/or the use of external control elements.
Thermal compensation can be provided via the use of thermal compensating birefringent elements or the arrangement of external components with respect to the series of at least one phase retardation plate or some combination of both. The surfaces of the series of at least one phase retardation plate, thermal compensating birefringent elements, the external elements, and the external control elements may be anti-reflection coated where appropriate to improve efficiency.
In general, in one aspect, the invention features an optical system including: a source which during operation generates two nonparallel propagating source beams; and a retarder element positioned to receive the two nonparallel propagating source beams and convert them into two nonparallel propagating output beams that are polarized substantially orthogonal to one another.
The optical system may include any of the following features. The nonparallel propagating source beams are diverging. The nonparallel propagating source beams are converging. The nonparallel propagating output beams are diverging. The nonparallel propagating output beams are converging. The retarder element is a retardation plate having substantially parallel entry and exit faces. The system further includes an additional retarder element positioned along a path defined by the source and output beams. The additional retarder element is positioned to receive the output beams and change their polarizations. The additional retarder is a half waveplate. The additional retarder is a quarter waveplate. The additional retarder element compensates for temperature dependent changes in the birefringence of the first mentioned retarder element. The additional retarder element is separate from the first mentioned retarder element. The additional retarder element is positioned to receive the nonparallel propagating output beams and generate nonparallel propagating output beams that exit from the additional retarder. The system further includes a third retarder element positioned to receive the nonparallel propagating output beams and generate substantially coextensive and collinear output beams that exit from the third retarder. The third retarder element is a birefringent prism. An optical axis of the retarder element lies substantially in a plane defined by the source beams. The retarder element is uniaxial. The optical frequencies of the two nonparallel propagating source beams differ from one another. The source includes: a laser generating a single-frequency, polarized beam; and a Bragg cell positioned to receive a beam derived from the polarized beam and generate the two nonparallel propagating source beams having optical frequencies that differ from one another. The source further includes: a source retarder element positioned to receive the beam derived from the polarized beam and transform it into ordinarily-polarized and extraordinarily-polarized beams, wherein immediately before exiting the source retarder element, the ordinarily-polarized and extraordinarily polarized beams generate a composite beam formed by a pair of overlapping beams, and wherein the Bragg cell is positioned to receive the composite beam and generate the two nonparallel propagating source beams having frequencies that differ from one another. The source further includes: a beam expander positioned to receive the beam derived from the polarized beam and expand the size of the polarized beam, and wherein the Bragg cell is positioned to receive the expanded beam and generate the two nonparallel propagating source beams having frequencies that differ from one another. The beam derived from the polarized beam is the polarized beam. The system further includes a beam contractor positioned to receive the nonparallel propagating output beams and contract the size of the nonparallel propagating output beams. The system is part of a distance measuring interferometry system, which also includes: an interferometer that directs at least a portion of one of the output beams along a reference optical path and at least a portion of the other of the output beams along a variable optical path and thereafter combines the portions of the output beams into a signal beam; and a detector for measuring an intensity of the signal beam. The detector includes a polarizer for producing a polarized signal beam having a polarization different from the polarizations of the output beams and the intensity of the signal beam measured by the detector is an intensity of the polarized signal beam. The interferometry system further includes measurement electronics for determining changes in the variable optical path from the measured intensity.
In general, in another aspect, the invention features a system including: a source which during operation generates first and second source beams propagating along nonparallel directions; and a retarder element positioned to receive the first and second source beams and to transform each of the first and second source beams into an ordinarily-polarized beam and an extraordinarily-polarized beam, wherein immediately before exiting the retarder element, the ordinarily-polarized and extraordinarily-polarized beams generated from the first source beam differ in optical phase by a first amount and the ordinarily-polarized and extraordinarily-polarized beams generated from the second source beam differ in optical phase by a second amount and wherein the first and second amounts differ by a value that is substantially equal to xcfx80 radians (modulo 2 xcfx80).
The system may include any of the following features. The first amount is substantially equal to xcfx80 radians (modulo xcfx80). The first amount is substantially equal to xcfx80/2 radians (modulo xcfx80).
In general, in another aspect, the invention features a system including: a source which during operation generates first and second source beams propagating along nonparallel directions; and a retarder element positioned to receive the first and second source beams and transform each of the first and second source beams into overlapping ordinarily-polarized and extraordinarily-polarized beams, wherein upon exiting the retarder element the overlapping portions of the ordinarily-polarized and extraordinarily-polarized beams produced from the first source beam form a first output beam and the overlapping portions of the ordinarily-polarized and polarized beams produced from the second source beam form a second output beam and wherein the first and second output beams are polarized substantially orthogonal to one another.
The system may include any of the following features. An optical axis of the retarder element lies substantially in a plane defined by the first and second source beams. The optical axis makes an angle of about 45xc2x0 with an axis collinear with the first source beam.
In general, in another aspect, the invention features a system including: a retarder element positioned to receive two nonparallel propagating input beams and convert them into two nonparallel propagating output beams that are polarized substantially orthogonal to one another; and a birefringent prism positioned to receive the two nonparallel propagating output beams from the retarder element and convert them into two substantially parallel optical beams that are polarized substantially orthogonal to one another.
The system may include any of the following features. The retarder element and the birefringent prism are integral with one another. One of the two nonparallel propagating output beams propagates within the birefringent prism as an ordinarily polarized beam and the other of the two nonparallel propagating output beams propagates within the birefringent prism as an extraordinarily polarized beam. The birefringent prism is made from a material in the group consisting of LiNbO3, KDP, quartz, and TeO2. The retarder element is made from a material in the group consisting of LiNbO3, KDP, quartz, and TeO2. The birefringent prism is a Wollaston prism. The system further includes a waveplate positioned between the retarder element and the birefringent prism.
In general, in another aspect, the invention features a system including: a source which during operation generates first and second source beams propagating along nonparallel directions; a retarder element positioned to receive the first and second source beams and produce first and second intermediate beams; and a birefringent prism positioned to receive the first and second intermediate beams and transform each of the first and second intermediate beams into ordinarily-polarized and extraordinarily-polarized beams, wherein the prism has a shape and a birefringence that causes the ordinarily-polarized beam produced from the first intermediate beam to produce a first output beam and the extraordinarily-polarized beam produced from the second intermediate beam to produce a second output beam, wherein the first and second output beams exit the prism substantially parallel to one another and wherein the combined energy of the first and second output beams is greater than half of the combined energy of the two source beams. In some embodiments, the polarizations of the first and second source beams are substantially the same.
In general, in another aspect, the invention features a system including: a source which during operation generates two nonparallel propagating source beams that are polarized substantially parallel to one another; a retarder plate positioned to receive the two nonparallel propagating source beams and produce two nonnparallel propagating intermediate beams, wherein the retarder plate has a thickness, birefringence, and orientation that causes the two nonparallel propagating intermediate beams to be polarized substantially orthogonal to one another upon exiting the retarder plate; and a birefringent prism positioned to receive the two nonparallel propagating intermediate beams and produce two output beams that are polarized substantially orthogonal to one another, wherein the prism has a shape and a birefringence that causes the two output beams to be substantially parallel to one another.
The system may have any of the following features. The system further includes a half waveplate positioned between the retarder plate and the birefringent prism to change the polarizations of the two nonparallel propagating intermediate beams. An optical axis of the retarder plate is substantially orthogonal to an optical axis of the birefringent prism.
In general, in another aspect, the invention features a method including the steps of: generating first and second beams which propagate along nonparallel directions; separating each of the first and second beams into overlapping ordinarily-polarized and extraordinarily-polarized beams; retarding the extraordinarily-polarized and ordinarily-polarized beams produced from the first beam relative to one another, wherein the overlapping portions of the extraordinarily-polarized and ordinarily-polarized beams produced from the first beam form a first output beam; and retarding the extraordinarily-polarized and ordinarily-polarized beams produced from the second beam relative to one another, wherein the overlapping portions of the extraordinarily-polarized and ordinarily-polarized beams produced from the second beam form a second output beam, and wherein the first and second output beams are polarized substantially orthogonal to one another.
The method may include any of the following features. The method further includes the step of making the first and second output beams propagate parallel to one another. The method further includes the step of making the first and second output beams substantially coextensive with one another. The first and second output beams have optical frequencies that differ from one another.
The invention has many advantages. It provides systems and methods for efficiently generating two substantially coextensive and collinear beams having orthogonal polarizations. In particular, the present invention has a conversion efficiency of nominally 100% for conversion of input intensity into intensities of two orthogonally polarized exit beam components, and in certain end use applications the intensity of each of two orthogonally polarized exit beam components may be adjusted to nominally 50% of the input intensity.
The system is also compact and requires relatively few optics. Additional optics can be included to optimize the overlap of the orthogonally polarized beams and to compensate for temperature-dependent changes in the birefringence of the retarder elements.
Furthermore, in other embodiments, the invention provides an apparatus for generating orthogonally polarized beams of different frequency with a predetermined angle of divergence between them and a predetermined lateral separation between their energy flux profiles.
Also, the invention can provide the source beams for a heterodyne detection distance measuring interferometry system. Such systems can provide the precise position and orientation of objects being processed, such as in semiconductor wafer processing. Moreover, because of the efficient generation provided by the invention, a single laser source in the interferometry system can drive interferometric distance measurements over a large number of measurement axes. In some embodiments, the invention also uses an acousto-optic modulator to generate a relatively large frequency difference (e.g., about 20 MHz) in the orthogonally polarized beam. This large bandwidth enables relatively fast scan speeds in the distance measuring apparatus.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.